The present invention is related to telephone terminal equipment. More specifically, this invention is a method and apparatus for the transmission of digital data of relatively low frequency between the common equipment and the station equipment in a telephone system installation, and for the detection of such digital data.
Known circuit arrangements for data transmission between a key service unit (KSU) and key telephone station sets employ a conductor pair separate from the power supply conductors. Techniques for super-imposing digital data signals on the power supply conductors are also known, but suffer from the disadvantage of noise induced by external sources.
It is the object of the present invention to enable the transmission and reception of digital data signals on the same conductor pair employed to transfer power from a key service unit to the station sets in a key telephone system without the disadvantages of prior art circuits.